In convention scanners which utilize a document handler to convey original documents to the scanning or input area, some type of mechanism is employed to register the document at a required exposure position. These mechanisms position the document more accurately and reduce the skew. In the conventional scanners, mechanical hardware is utilized to register the document for proper scanning.
An example of a conventional registration device is disclosed in the Xerox Disclosure Journal, Vol. 12, No. 1, entitled, “IMPROVED PERFORMANCE OF A DOCUMENT REGISTRATION FIGURE/PLATEN INTERFACE,” the entire contents of which are hereby incorporated by reference. In this conventional registration system, a copier scans the image through a glass platen. The registration system also includes a plastic ramp which adjusts the height of the glass platen by means of an adjusting screw. The plastic ramp also serves as a guide for registration fingers to move from an out of registration position to a registration position. In the registration position, the registration fingers butt up against the edge of the platen and protrude above the top surface such that a document conveyed on top of the surface of the platen is stopped at the fingers in the required registration position.
Another example of a conventional registration and deskew device is disclosed in Xerox Disclosure Journal, Vol. 10, No. 2, entitled, “LEADING EDGE DESKEW AND REGISTRATION DEVICE,” the entire contents of which are hereby incorporated by reference. The Xerox Disclosure Journal article discloses a device which provides both deskewing and registration. In the registration system, two paper guide plates are utilized. An original document moves along a first guide plate towards a second guide plate by a means of a conveyor system or pinch rollers. The document stops at a wait station as the leading edge of the document enters an area between the first guide plate and the second guide plate. As the document enters this area, the leading edge registration device is in the raised position such that two guides on the registration device project up into the area between the first paper guide plate and the second guide plate. The leading edge registration device then moves a fixed amount opposite the direction of the travel of the document so as to deskew the document and place the document into a proper registration position. After the deskewing motion, the leading edge registration device lowers out of the paper path and the document can continue from the first paper guide plate to the second paper guide plate.
A drawback associated with conventional registration systems is that conventional registration systems require additional mechanical hardware in the paper path so as to physically register the document in the proper scanning position. This utilization of mechanical devices to physically register the document for proper scanning is a relatively slow solution which cannot be readily utilized in a high speed scanner. Moreover, such mechanical systems cannot be utilized in a centered registered constant velocity transport scanner since the exact fast scan location of the input can vary from document to document. Therefore, for a registration system to be utilized in a high speed scanner or in conjunction with a centered registered constant velocity transport system scanner, the registration system must be able to register the input of the document quickly, dynamically, variably, and accurately.
Automatic document detection is intended to locate “any rectangular piece” of document placed “anywhere” on the platen. Such a process requires an initial prescan to locate edges (black->white transition followed by white->black transition). With the detection of the corners of the document with respect to the platen edges, a second scan is performed to image the appropriate region (either the outer bounding box containing the entire document, or the largest possible rectangular box that could be enclosed within the four corners to avoid any black borders). The document size obtained during prescan enables the machine to either setup reduction/enlargement parameters to fit onto a preselected output paper or to select the appropriate output paper to fit 100% of the input image.
Although this process is available, it desirable to have a process which does not rely on the assumption of the input document's size. The present invention is directed to a method for detecting all the four corners of a document placed on a platen. Unlike the process mentioned in U.S. Pat. No. 5,528,387, the present invention makes no assumption with respect the input size of the document to determine the third and fourth corners (C2, C3) of the input document.